This invention relates to a cathode ray display tube comprising an envelope containing means for generating an electron beam and a phosphor screen, and having a transparent faceplate through which the display produced by the screen is visible.
The invention is concerned particularly, although not exclusively, with a so-called flat cathode ray display tube generally of the kind described in British Patent Specification No. 2101396, corresponding to U.S. patent application Ser. No. 830,388, filed Feb. 14, 1986, now U.S. Pat. No. 4,737,690, which has a rectangular, box-like, metal rear housing covered by a substantially flat glass faceplate. In this tube, an electron gun directs a low-energy electron beam parallel to the screen and faceplate. The beam is then turned through 180.degree. by means of a reversing lens at one end of the tube before being deflected to scan in raster fashion over the input side of an electron multiplier arranged parallel to, and spaced from, the screen. The beam undergoes electron multiplication within the multiplier and is then accelerated onto the screen by an accelerating field established between the output side of the multiplier and the screen in order to produce a display on the screen.
An advantage of this tube, made possible by the provision of an electron multiplier, is that the electron beam, before reaching the multiplier, need only be of comparatively low-energy, for example, a low voltage, low current beam having an acceleration voltage less than 2.5 kV and typically around 600V. Consequently deflection of the beam to achieve raster scanning, which is carried out prior to the multiplier by means of deflection electrodes, is rendered compact and simpler as only relatively small electrostatic fields are then necessary, the desired brightness of the display being achieved by the final acceleration of the current-multiplied beam emanating from the multiplier.
However, the use of a low energy electron beam in this way means that operation of the tube is particularly susceptible to the effects of extraneous magnetic fields. The sensitivity of the tube to ambient magnetic fields penetrating the envelope can be such that even the earth's magnetic field may interfere with the course of the electrons comprising the low-energy beam.
The problem of the influence of such magnetic fields on the operation of more conventional forms of cathode ray display tubes using a high energy electron beam and having a generally conical shape envelope has been recognised for some time. It has therefore been suggested previously to surround much of the tube's conical envelope with soft magnetic material which provides useful magnetic screening from fields at rights angles to the main axis of the tube. However, with such an arrangement there remains a very large plane, the faceplate of the tube, which is not screened, especially from magnetic field components parallel to the axis of the tube.
The box-like rear housing of the aforementioned flat display tube can be screened in a similar manner by surrounding it with magnetic shielding material to alleviate to some extent the problem of extraneous magnetic fields influencing undesirably the trajectory of the electron beam within the tube. However, the faceplate of this tube comprises a relatively large area of the tube's envelope and the effects of magnetic fields entering the envelope through the faceplate are still very significant. The problem is increased because of the low-energy nature of the electron beam used in this tube and its greater susceptibility to magnetic fields, and also because much of the beam's path of travel is in directions parallel to the plane of the faceplate.
It is an object of the present invention, therefore, to eliminate, or at least reduce substantially, magnetic fields from entering through the faceplate of the cathode ray display tube and particularly, but not exclusively, the faceplate of the aforementioned flat kind of display tube using a low energy electron beam.